Waveform generator with audio tone control



Aug. 26, F. BRQGAN WAVEFORM GENERATOR W ITH AUDIO TONE CONTROL 3 Sheets-Sheet 1 Filed Jan. 17, 1966 v INVENTOR. FRA/Vam4am 26, 1969 F. A. BROGAN WAVEFORM GENERATOR WITH AUDIO TONE CONTROL 3 Sheets-Sheet 2 Filed Jan. 17, 1966 Aug. 26, 1969 F. A. BROGAN WAVEFORM GENERATOR WITH AUDIO TONE CONTROL 3 Sheets-Sheet 5 Filed Jan. 17, 1966 INVENTOR. F/PI/l/GVJ' 14. 54004 BY Q fa m kiwi,

i fi wii United States Patent 3,464,030 WAVEFORM GENERATOR WITH AUDIO TONE CONTROL Francis A. Brogan, San Antonio, Tex., assignor to the United States of America as represented by the Secretary of the Air Force Filed Jan. 17, 1966, Ser. No. 521,224 Int. Cl. H03c 1/02 US. Cl. 332-3 2 Claims ABSTRACT OF THE DISCLOSURE A circuit for transferring tones of oscillators to a speaker including photo-conductive tubes in series with a transformer system and a rotating disc with slits interposed between the photo-conductive tubes and their light sources.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to a waveform generator and more particularly to an apparatus for keying audio tones.

This invention consists of an apparatus for keying a pure tone audio oscillator with controlled rise and decay time for use in pure tone audiometry. The apparatus has a photo-conductive cell connected in series with two transformers such that when the light beam to the photoconductive cell is interrupted, the photo-conductive cell places a high resistance between the first and second transformer. The resistance is changed in a gradual manner depending upon the rise-decay time of the photo-conductive cell and the shape of the moving slit in a rotating wheel placed between the light source and the photo-conductive cell. The purpose of the two transformers is to achieve optimum operation between the source and the load. By the means disclosed, various patterns of tones can be generated differing in amplitude and silent interval separation and in the envelope shape.

Previously, one method for controlling the onset and offset envelope of an oscillator was by using an amplifier having a shaped grid bias voltage that would shape the envelope of the audiometric tone from the oscillator by an RC circuit that controlled the characteristics of the applied grid bias. Another means was by the use of an LC oscillator whose plate voltage was keyed on and off with an appropriately shaped envelope of voltage which was used to control the envelope of the tone presented. The invention described here which uses the principle of a photo-conductive cell in combination with a rotating disk and a like source is simpler, quieter, more economical and durable. Also the envelope is capable of being shaped in a more uniform manner.

An object of this invention is therefore to provide an improved apparatus for shaping the envelope of an audiometric tone amplitude.

Another object is to provide a means for presenting tones with shaped envelopes but of different or equal amplitudes.

Still another object is to provide a means for varying the silent interval between presentation of tones.

It is yet another object to provide a means for achieving very small controlled incremental tone pips superimposed on a background level tone for use in diagnostic hearing tests.

These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings wherein:

FIGURE 1 is a diagrammatic view of a basic form of the invention;

FIGURE 2 shows the electrical circuit associated with the basic form of FIGURE 1;

FIGURE 3 is a diagrammatic view of an embodiment of the invention;

FIGURE 4 shows the electrical circuit associated with the embodiment of FIGURE 3;

FIGURE 5 is a waveform diagram demonstrating the output obtained with the apparatus of FIGURES 3 and FIGURE 6 shows a diagrammatic view of another embodiment of the invention; and

FIGURE 7 shows an alternate electrical circuit that can be employed in the invention.

We now refer to FIGURE 1 which shows the basic form of this invention as an audiometric tone interruptor circuit. The sum of the desired on period and the off period determines the required speed of motor 11. As an example, if it is desired to have an on time of one second and an off time of one second, the total time for the motor to rotate disk 12 would be two seconds; therefore, a motor having a speed of 30 rpm. should be selected. Slit 13 having an arc length of should be cut from disk 12 which is connected to shaft 16 of motor 11. The ends of slit 13 should be shaped to achieve the required rise time since the change in resistance of photo-conductive cell 15 is proportional to the amount of light striking it from light source 14. Photo-conductive cell 15 should be located on one side of disk 12 and light source 14 on the other. If desired, disk 12 can be made of transparent material and an opaque paper or cardboard glued to disk 12 with the desired number and size of slits or openings. Another method of controlling the change of intensity of light rather than by slit shaping would be to use a film strip or other material with a controlled light transmission loss material.

Referring to FIGURE 2 showing the electrical portion of the invention, oscillator 21 is coupled to earphones 22 by transformers 23 and 24. A speaker could also be used. The output is controlled by the resistance of photo-conductive cell 15 which is determined by the amount of light falling upon it. In addition, the output is controlled by adjustable attenuator 25.

FIGURES 3 and 4 show an embodiment of the invention for keying two shaped envelopes of tones that have an adjustable ,difference of intensity by using two cells 32 and 33 and two light sources 34 and 35. As an example of this embodiment, if it is desired to have the first tone on for 300 milliseconds followed by a silent interval of 300 milliseconds and a second tone on for 300 milliseconds followed by a silent interval of 1100 milliseconds before the start of the first tone again as shown in FIGURE 5. Since the total time period is two seconds, motor 31 should have a speed of 30 rpm. and slit 36 of disk 37 should have an arc length of 54 degrees. The combination of light and photo-conductive cells of the first combination should be spaced twice 54 or 108 apart from the second combination adjacent to the disk. As can be seen in FIGURE 4, photo-conductive cells 32 and 33 are connected in parallel. Variable resistor 41 is in series with photo-conductive cell 33 and the combination is in series with transformers 42 and 43. The operation is such that when photo-conductive cell 32 is illuminated the tone goes through transformer 42 which as an example could be 500 ohms. When photo-conductive cell 32 is illuminated the maximum amount of energy is applied to transformer 43 with some loss in the photo-conductive cell. When photo-conductive cell 33 is illuminated, it drops to approximately the same value as photo-conuductive cell 32, but due to adjustable resistor 41 in series, photo-conductive cell 33 does not deliver the same output to the load but is some value less, which is adjustable by series resistor 41. When photo-conductive cells 32 and 33 are not illuminated the parallel resistance is in series with transformers 42 and 43 and since this resistance is very high at about 500 megohms, very little energy will pass the circuit to the audio output shown as earphones 45. Neglecting the capacitance of the wiring and the cells, the on-off ratio using 500 ohm transformers should be in the order of ten to the fourth. The output change as a result of varying the position of variable resistor 41 can be determined in increments either by calculation or measurement.

Another embodiment of this invention as shown in FIGURE 6 is for superimposing one tone increment on a second tone envelope. This embodiment would be for the purpose of providing means for a special diagnostic test that requires the threshold determination of a tone envelope that occurs during the presentation of a second tone envelope. The pair of photo-conductive cells 51 and 52 and light sources 58 and 59 are located such that when slits 53 and 54 of disk 55 are both open, the time distance between cells is zero; that is, slits 53 and 54 are concentric on the same axis. As an example, assume that the interrupted background tone is to be on for 1.5 seconds and off for 1.5 seconds. Therefore, the speed of motor 56 should be 20 r.p.m. and the inner slit should have an arc length of 180. If it is desired to have the superimposed tone occur at a maximum level after the background tone has been on for 650 milliseconds and have the superimposed tone last at a maximum amplitude for 200 milliseconds, outer slit 53 should have its maximum width occur at .65 120 less the time constant of the photo-conductive cell from the maximum width starting point of inner slit 54. The shape of the slit from this point will be made with an appropriate curve to zero from calculations using the desired rise time and photoconductive cell data or determine experimentally by changing the slit opening and observing the results on a storage type oscilloscope. The maximum length of this slit would last for .2 120 and the slit could be made to close at the same rate as determined by either method above.

An alternate electric circuit is shown in FIGURE 7 where two sets of transformers are utilized, one set comprising transformers 61 and 62 connected to first tone source 77 and the other set comprising transformers 63 and 64 connected to second tone source 68 which can be the same or different. Photo-conductive cells 65 and 66 are connected to one set each. Potentiometers 67 and 68 are connected in series with each of the cells for the purpose of equalizing the two tones. The output of each transformer set is amplified by amplifiers 69 and 70 and the outputs thereof are fed respectively to attenuators 71 and 72 whose outputs are then mixed in a mixing pad comprising resistors 73 and 74. The resulting audio tones are audible at earphones 76. VU meter 75 can be placed across the inputs of attenuators 71 and 72 and then potentiometers 67 and 68 can be adjusted to make each VU meter read the same. When attenuators 71 and 72 are equal, the short tone will be 6 db greater than the background tone. By calculation and measurement the size of the short tone above the background tone can be determined from the db values of the short tone attenuator.

What I claim is:

1. A keying apparatus for obtaining a sequence of audio tones comprising:

(a) an oscillator;

(b) an audio output means;

(0) means for transferring the oscillator output to the audio output means, the transfering means includmg:

(1) a first transformer having a first and second coil with the first coil connected to the oscillator;

(2) a second transformer having a first and second coil with the second coil thereof connected to the audio output means and the first coil thereof connected to the second coil of the first transformer; and

(3) a plurality of parallel-connected photosensitive devices interposed between and in series with said first and second transformers, at least one of said plurality having a variable resistor in series therewith;

(d) a rotating disc having an aperture and positioned for sequentially scanning each of said plurality of photo-sensitive devices; and

(e) a plurality of light sources positioned to project light on said photosensitive devices during said sequential scanning upon the photo-conductive cells upon the occurrence of the aperture being in alignment with the photo-conductive cells and the light sources.

2. A keying apparatus according to claim 1 which further includes an attenuator interposed between the audio output means and the second transformer.

' References Cited UNITED STATES PATENTS 2,219,676 10/1940 Barber 332-3 2,241,533 5/1941 Bliss 332-3 2,730,567 1/ 1956 McConnell 332-3 X 1,822,061 9/1931 Roberts 250206 X 1,871,994 8/1932 Iams 250206 X 2,012,573 8/1935 Long 250206 3,222,529 12/1965 Askowith 250206 X 2,014,741 9/1935 Lesti 841.18 2,067,980 1/1937 Nicolson 841.18 2,169,842 8/1939 Kannenberg 84-1.18 X 2,520,138 8/1950 Frink 250233 X 2,576,760 11/1951 Jones et al. 841.l8 2,747,797 5/1956 Beaumont 250233 X 3,110,009 11/1963 Bolton et al 250233 X ROBERT SEGAL, Primary Examiner U.S. Cl. X.R. 

